Referring to FIG. 1 current Doppler radar systems 5 generally employ a transmitter emitting from radio frequency sources such as a magnetron or other acceptable radio frequency source utilized in Doppler radar systems. Radio frequency signals from transmitter 10 are transmitted to Doppler radar antenna 13 via various couplers 11 and passing through circulators 12 that protect receiver components housed in typical receiver enclosure 16. In some instances, slip rings and pedestal hub conduits 9 are required for transmissions and communications to be transmitted from the transmitter 10 to the radio frequency enclosure 16. Cross-guide coupler 11 also provides a low level sample of the transmitter burst frequency that is propagated to mixer 29. Essentially, cross-guide coupler 11 serves as a signal “tap” to allow for extraction of the transmission burst signal. Essentially, also, a sample of the transmitter frequency is taken.
A transmitter receiver limiter 14 provides additional protection functions to the receiver enclosure 16 and, in conjunction with circulator 12, provides full protection for the receiver 16 from being overwhelmed by transmissions from transmitter 10. Ethernet hub/switch 23 also provides a communications function to allow receiver 16, and its components installed therein, to be able to communicate outside of the radar system 5 and for communications with the antenna, transmitter, and other radar components that may be outside of the radio frequency receiver enclosure 16. Receiver enclosure 16 includes electronic mixing component 17, intermediate frequency digitization (i.e. conversion from analog to digital and vice versa) and associated power supply functions 21, and signal processing module 18. Ethernet hub/switch 23 may or may not be within radio frequency enclosure 16.
Test signal generator 51 provides a known test signal at a known frequency to be utilized within internal receiver components 17 for testing purposes. Radio frequency switch 24 allows for test signals generated by test signal generator 51 to be propagated through other components, as desired, as opposed to the nominal propagation of reflectivity signals to receiver 16. Low noise amplifier 27 amplifies reflectivity signals propagated through RF switch 24 and selectively filtered through filter/isolators 28 into mixer 37. Filters 28 typically operate in the GHz range of frequencies, but signals entering mixer 37 from filter 28 are mixed with stable local oscillator 36 to generate a mixed signal into bandpass filter 38 at an intermediate frequency. Bandpass filter 38 allows passage of signals within the 60 MHz range, which are then amplified by intermediate frequency amplifier 41 and attenuated at 42. Attenuator 42 is necessary in order to obtain correct power and frequency levels in order for intermediate frequency digitization in component 46 so that signal processing module 18 may process those digitized signals.
In addition to processing Doppler reflectivity signals, signal processing module 18 controls, via Ethernet hub/switch 23, the initiation of a firing sequence from transmitter 10. However, radio waves created by transmitter 10 are transmitted at a frequency largely dependent upon the physical dimensions and ancillary electronics powering the transmitter 10. For example, a resonant cavity magnetron transmitter produces transmission signals for propagation outside of radar system 5 via antenna 13 to produce reflectivity data. Firing signals, usually based upon a local oscillator and associated electronics, control its operation. But, depending upon the physical dimensions of the magnetron unit, outputs typically vary by a few percent in their frequency output, depending upon temperature and other environmental factors. As the frequency from transmitter 10 varies, a digital signal processor 20 of module 18 measures the transmitter frequency based upon inputs from variable attenuator 34. Elements 31, 33 and 34 all receive mixed intermediate frequency signals via mixer 29 based upon signal inputs from cross-guide coupler 11 and then pass those signals to digitizer 46. As digital signal processor 20 receives those signals, it processes and analyzes the frequency of transmitter 10.
Conventionally, as signal processing module 18 encounters frequency variations from transmitter 10, it sends analog or digital signals, depending upon the type of STALO 36 designed into system 16, to STALO 36 via RS-232 (or equivalent) feedback path 53 in order to alter the STALO 36 output to compensate for transmission frequency variance and for re-aligning the intermediate frequency entering digitizer 46 to keep intermediate frequency at 60 MHz. Digital STALO 36 typically incorporates external controls to allow for variance in the digital STALO output to mixers 29 and 37. Essentially, the magnetron 10 output signal is sampled at coupler 11 and synchronized with the receiver 16 to whatever frequency the source 10 is generating.
To measure the slight change in the IF signal frequency to process the signal for radar reflectivity data, the IF signal is typically first shifted downward in frequency to near zero frequency (often referred to as “base-band processing”), by mixing with another frequency-stable signal (referred commonly as the “COHO”, for “coherent oscillator”). The frequency of the COHO is normally adjusted, by phase-locking or other means, to convert the IF frequency resulting from a stationary target to a frequency of exactly zero Hz. The frequency conversion to base-band may be accomplished by mixing the IF signal with the COHO oscillator, to produce a complex zero-IF signal (analog I and Q processing), or, in more modern “digital” receivers, by digitizing the IF signal directly (using a stable digitizing clock effectively as the COHO) and then using mathematical techniques to shift the center frequency of the digitized signal to a zero-frequency base-band IF.
The accuracy and stability of the two frequency conversion steps, first the conversion to an IF and then the conversion to a base-band, are highly critical to the technical performance of the Doppler weather radar. If the frequency or phase of either the STALO or COHO fluctuates during the reception period, or from one transmitter pulse to the next, phase errors are introduced into the digitized base-band signal for all types of receivers, whether analog I/Q receivers or digital receivers. These phase errors have the effect of spreading the spectrum of the received signal over the pass-band of the receiver (usually defined by the Nyquist interval appropriate to the sampling rate of the measurement, which in most cases is equal to the radar pulse rate). While these errors may affect the accuracy of the Doppler relative velocity measurement, more importantly they adversely affect the ability of the radar to discriminate between precipitation signals and signals reflected from ground targets (called “clutter”, from the effect that persistent ground reflections have in masking the observation of weaker signal returns from precipitation).
This baseband processing procedure is relatively straightforward for fixed-frequency transmitters, such as Klystrons, as modern means are available for generating fixed-frequency oscillations of the required stability. However, compensation of transmitter signals for the most common microwave weather radar transmitter tube, the magnetron, is problematic. The difficulty results because the frequency generated by a magnetron is not closely controlled, but is subject to both short-term and long-term frequency changes as the tube temperature, the applied voltage, the nearby magnetic field, or other physical parameters change with time. Thus magnetron radar systems must be designed to provide means for tracking the magnetron frequency changes, and for altering the conversion oscillators accordingly, to keep the received signal well within the receiver pass-band.
Commonly used techniques for altering the digital STALO 36 frequency, namely changing the parameters controlling the synthesizer or phase-lock loop, produce instabilities in frequency and phase of the STALO 36 for a period of time dependant upon the dynamics of the STALO control loop, or the COHO, or both. But, these instabilities have exactly the same negative effects on the performance of the Doppler radar system as compared with the instabilities in the transmitter frequency of a magnetron, and hence the ability of the radar system to discriminate against clutter targets is seriously degraded during the period when the STALO 36 is being altered from one frequency to another.
Fortunately, as digital STALO technology has increased to allow for much higher precision in digital isolation outputs as well as improved stable oscillator operation by providing precision inputs to transmitter 10, and upon the tuning of coherency in those oscillators, receivers 16 now have the ability to be much more stable during reception and analysis of reflectivity signals reflecting back from objects of interest received by antenna 13. However, in order to take full advantage of this increased receiver stability, it is necessary that the STALO frequency remain constant and not be varied to track the transmitter frequency drift.
Therefore what is needed is a system and method to obviate variations in a digital STALO's operation for a radar receiver so that continuous stability of a pre-determined frequency as can now be provided to a radar receiver may be incorporated and adjustments to compensate for fluctuations in the transmitter frequency may be accomplished using mathematical processing techniques to compensate for variation in transmitter frequency during baseband processing of radar reflectivity signals.